Placement unit for mounting electric components onto substrates

ABSTRACT

The invention relates to a placement unit including a rotatable holder for electric components that can be placed on a substrate. The holder is connected to a rotary drive of the placement device by a separable coupling which includes coupling elements that can be uncoupled in an axial direction. When the component is placed on the substrate, the coupling is completely separated, thus uncoupling the holder fully from the rotary drive. The holder is prestressed axially towards the component against the rotary drive by an adjustable pressure device, the coupling surfaces separating from one another by an axial application force that acts on the holder and prevails over the axial pre-stress. The application force can thus be adjusted individually in a more precise manner for different components.

The invention relates to a placement unit for mounting electriccomponents onto substrates, the placement unit having a housing, abuilt-in, rotatable holder for the components and, mounted within thehousing, a rotary drive for the holder, said holder being connected in atorsionally rigid manner to the rotary drive via an axially elasticcoupling.

Such a placement unit was disclosed for example in document U.S. Pat.No. 6,178,621 B. Such a placement unit is part of a pick-and-placesystem in which electrical components are made available in a pickingarea and placed in a component placement area on an electrical printedcircuit board. A pneumatic buffer having modifiable operating pressureis provided between the rotary drive and the component holder. Thisbuffer makes it possible to set the application forces to differentmaximum values, depending on the components concerned.

Furthermore document WO 03/001572 A discloses a placement unit in whicha rotary drive is connected in a torsionally rigid manner to the holdervia metal bellows which are not only laterally displaceable but alsocompressible in the placement direction. At its inner end the holder hasan anchoring element to which electromagnets apply a magnetic force, thestrength of which can be preset. The magnetic force can be adapted tothe maximum permitted limits of the different components by altering thesupply of current to the electromagnets. The holder is moreover insertedin a radial arbor yoke which is designed as an air bearing and so has nostatic friction. When tuning the application force it is necessary totake into account not only the magnetic force, but also the springloading applied by the metal bellows, as well as the intrinsic weight ofthe holder. Limitation of the application force is particularlyimportant in the case of small, pressure-sensitive components.

In the case of the placement unit disclosed in document U.S. Pat. No.5,029,383, the placement tip is held loosely and virtually withoutfriction opposite to a holder, in an air bearing shaped like a hollowsphere, so that the placement tip can adjust during placement to theshape of the component that needs to be picked up. Then only theweight-loading of the placement tip has any effect on the component. Inorder to pick up a component, a partial vacuum is applied to the airbearing and the placement tip, as a result of which the placement tipcomplete with component is fixed to the holder.

Particularly in the case of components having a fairly large number ofconnections, however, a certain minimum force is needed in order to makesure that all connections are dipped in the solder. The permitted forcemay therefore lie within a narrow tolerance range.

The object of the invention is to maintain with great accuracy theapplication force immediately affecting a component.

This object is achieved by means of the invention having the featureswhich will emerge from claim 1. A separable coupling ensures that thecoupling elements are safely uncoupled as soon as the component isplaced upon the substrate. This complete uncoupling of the holder fromthe rotary drive at the moment of placement prevents any further forcesbeing applied to the holder from this direction. The starting assumptionis that a component does not continue to twist after placement,particularly in the presence of the usual viscous soldering pastes. Whensetting the application force, coupling forces and spring-loading forcescan be ignored, improving both the speed and accuracy of the setting. Inparticular there is no need for further control measures, for instancein order to compensate for an increase in spring loading. The rotarydrive can be designed as a direct drive of a motor shaft, for example,or simply as a drive shaft connected to a drive motor via for instance agear box.

Advantageous embodiments of the invention will emerge from claims 2 to11.

It needs only a small amount of torque to twist a component into acorrect placement position, and in fact such torque can be reliablytransferred by a slightly prestressed friction coupling. Acorrespondingly slight pre-stress also uncouples the rotation axes ofthe rotary drive and the holder from one another, so that said holdercan be accurately guided in a purposely provided pivot bearing,unaffected by the rotary drive.

In the embodiments according to claims 3 and 4 the holder can be changedquickly. Such holders can be provided with different front ends, adaptedto the components that have to be directly picked.

However, it is also possible to pick components by means of a speciallyadapted suction pipette which can be quickly changed and fixed to theadapter on the holder.

In the embodiment according to claim 6 the angular rotation of theholder can be checked in the immediate vicinity of a component.

In the embodiment according to claim 7 the performance of thepick-and-place device is appreciably increased.

The embodiment according to claim 8 is intended to enable theapplication force to be checked even more accurately. The hollow,thin-walled, cylindrical holder can be made so light that its intrinsicweight is far less than the required application force and any bulkforces which occur remain correspondingly low, so that for the most partthe application force is defined by the adjustable pressure device.After the placement of a component on a substrate, it is usual to changethe vacuum to a short pressure pulse, in order to ensure that thecomponent adheres securely to the substrate. It is further customary tofeed the pneumatic line to the suction tip through a hole drilled in thecenter of the holder. Enlarging the interior of the holder results in abuffer volume that delays the transfer of the pressure pulse and wouldtherefore reduce placement performance. By making sure that thepneumatic line to which the invention relates is separated from the saidinterior, the internal volume is appreciably reduced and the vacuum atthe holder tip can very quickly be turned into a deliberate pressurepulse.

The invention according to claim 9 is likewise intended to enable theapplication force to be set even more accurately. The air bearing iscompletely free from hysteresis and friction in both the direction ofrotation and the direction of axial movement. The holder has no need offurther supporting elements which could increase its mass. Axialpressure actuation by means of pressurized gas (produced by a pressuredevice) requires no additional coupling elements on the holder. The gaspressure is distributed evenly over the whole surface cross-section ofthe holder, so that the point at which forces are in equilibrium is inthe center and no tilting moments can occur. A pressure sensor can beused to set the gas pressure accurately to a preset value even beforeplacement. In a combination of this kind the slight mass of the holderis the only interference variable, but this can easily be overcome bycontrolling the rate of descent of the placement unit. In the event of ahigher weight of parts without spring suspension, as for instanceaccording to document U.S. Pat. No. 6,178,621 B mentioned above, apressure control device would be necessary during the placementprocedure, but is not needed according to the present invention.

The ventilation slot according to claim 10 effectively uncouples the airbearing pneumatically from the actuation pressure of the holder.

In the embodiment according to claim 11, the pressure volume above theholder is increased so that any increase in pressure when the holdermoves in is negligible.

The invention will be explained in greater detail below with referenceto an exemplary embodiment shown in the drawings in which;

FIG. 1 shows a longitudinal section through a placement unit accordingto the invention in its starting position,

FIG. 2 shows a partial section through said placement unit along theline II-II in FIG. 1 in an operating position,

FIG. 3 shows a section through said placement unit along the lineIII-III in FIG. 2.

According to FIGS. 1, 2 and 3 a carrier 1 of a placement head 2 ismaneuverable in two horizontal coordinate directions X and Y parallel toa substrate 3. The carrier is provided with a vertical linear guide 4 inwhich a placement unit 5 can be moved in a further coordinate directionZ by means of a lifting drive 28 of the carrier 1. An electrical rotarydrive 7, located in a housing 6 of the placement unit 5, engages on theunder side with a drive shaft 8 in a coaxially arranged piston-typeholder 9 intended for an electrical component 10. The holder 9 can berotated and moved lengthwise inside a cylindrical air bearing 11.

At its end facing the rotary drive 7, the mainly hollow, cylindricalholder 9 has an end wall 12 through which the drive shaft 8 projects. Onthe side of the end wall 12 facing away from the rotary drive, a washerdisk 13 is fixed on the drive shaft 8, and retains the end wall with theaid of laterally projecting aliform arms. In the operating positionshown in FIG. 1, the underside of the end wall 12 is under axialpre-stress on the upper side of laterally projecting arms of the washerdisk 13. The frictional force is strong enough to reliably transmit therotation of the rotary drive 7 to the holder 9, so that the component 10can be oriented and placed in the required position on the substrate 3.

The end wall 12 has an opening 15, the contour of which is adapted tothe washer disk 13, with play, and extends perpendicularly to thelengthwise direction of the washer disk 13. In a placement positionrotated through 90°, it is then possible to pass the end of the driveshaft 8, with the washer disk 13 attached, through the end wall 12. Thedrive shaft 8 is then rotated relative to the holder 9 until the lateralarms of the washer disk 13 are on rotary stops 14 (not shown in FIGS. 1and 3) of the end wall 12, so that the holder 9 with the drive shaft islocked in the manner of a bayonet fastening and prevented fromdescending in the Z coordinate direction.

Above the end wall 12 a closed hollow space 16 is formed in theplacement unit 5 and is linked to a connected compressed air line 17.The incoming compressed air constitutes a pressure device which pressesthe end wall 12 against the washer disk 13. The end wall 12 and thewasher disk 13 constitute the coupling elements of a friction clutch inwhich the sides facing one another act as coupling surfaces 18. Saidsurfaces are so strongly deformed by the pneumatic compressive forcethat they reliably transmit the slight torque required to rotate theholder 9 complete with the component 10.

The air bearing 11 is formed in a guide bushing 19 which extends overthe hollow space 16 and is provided with narrow radially drilled holes(not shown) that are connected to a further compressed air line 20.Arranged in the guide bushing 19 between this region and the upper endof the holder 9 is a surrounding ventilation slot 27 which prevents theunchecked spillage of waste air between the two compressed air regions,so that both regions remain free from mutual pressure interactions.

In the area of its guideway the holder 9 is in the form of a hollow,thin-walled cylinder, the interior 21 of which has a correspondinglylarge volume, so that the weight of the holder 9 can be kept very low.The interior 21 is pneumatically separated by a closed bottom plate froma vacuum line 22 for drawing the component 10 onto the holder 9. At itslower end the holder 9 has an adapter 23 to which is attached a suctionpipette 24, the function of which is assigned to the holder 9. However,it is also possible to design the holder tip itself as the suctionpipette, which will then draw up the component 10 directly.

Via the adapter 23 a disk shaped angle scale 25 is attached to theholder 9, said scale being detected by a stationary sensor 26 attachedto the housing 6. By this means the rotation of the holder 9 by therotary drive 7 can be measured in the immediate vicinity of thecomponent 10, so that a possible transmission error has no effect.

According to FIG. 2 the placement unit 2 is lowered by the lifting drive28 in the Z direction until the component 10 is placed on the substrate3. At this moment the holder 9 is pushed into the hollow space 16 suchthat the prevailing gas pressure defines the maximum application force.It is therefore advantageous for it to be possible to set thecontrollable gas pressure accurately beforehand. Different componentsrequire different application forces, which are obtained by applyingdifferent pressures. At the moment when the holder 9 moves in it isstopped and pressed into the remainder of the placement unit. Thisrelative movement can be detected by a suitable sensor, triggering theimmediate braking of the vertical movement. Immediately upon placement,the end wall 12 is completely separated from the washer disk 13, so thatno further bulk forces and coupling forces affect the holder 9.

REFERENCE CHARACTERS

-   -   x, y, z Coordinate direction    -   1 Carrier    -   2 Placement head    -   3 Substrate    -   4 Linear guide    -   5 Placement unit    -   6 Housing    -   7 Rotary drive    -   8 Drive shaft    -   9 Holder    -   10 Component    -   11 Air bearing    -   12 End wall    -   13 Washer disk    -   14 Rotary stop    -   15 Opening    -   16 Hollow space    -   17, 20 Compressed air line    -   18 Coupling surface    -   19 Guide bushing    -   21 Interior    -   22 Vacuum line    -   23 Adapter    -   24 Suction pipette    -   25 Angle scale    -   26 Sensor    -   27 Ventilation slot    -   28 Lifting drive

1. A placement unit for mounting electric components onto substrates,the placement unit comprising: means for moving perpendicularly to thesubstrate, a housing, a built-in, rotatable holder for the componentsand, a rotary drive for the holder mounted within the housing, theholder connected via an axially elastic coupling to the rotary drive ina manner which is at least temporarily torsionally rigid, wherein thecoupling comprises a separable coupling having coupling elementsarranged to be uncoupled in the axial direction, and that the holder ispre-stressed axially toward the component against the rotary drive by anadjustable pressure device, the coupling surfaces of the couplingelements form an axial stop for the holder, and the coupling surfacesseparate from one another by an axial application force that acts on theholder and prevails over the axial pre-stress.
 2. The placement unitaccording to claim 1, wherein the coupling is a friction clutch havingcoupling surfaces of the coupling elements extending perpendicularly tothe axis of rotation.
 3. The placement unit according to claim 1,wherein the coupling is arranged such that it can be rapidly separatedand the holder can be exchanged.
 4. The placement unit according toclaim 1, wherein the holder is mounted in the housing such that theholder can be moved axially, and the holder is connected in the mannerof a bayonet fastening to a drive shaft of the rotary drive.
 5. Theplacement unit according to claim 1, wherein the holder includes anadapter for attaching a detachable suction pipette for the component. 6.The placement unit according to claim 1, wherein the holder includes anangle scale and that a sensor for the angle scale is provided on thehousing.
 7. The placement unit according to claim 1, wherein a pluralityof individually controllable placement units are arranged in parallel ina movable placement head.
 8. The placement according to claim 1, whereinthe, cylindrical holder is mainly hollow and designed to be thin-walledin its cylindrical surface area, and the placement unit furthercomprises a vacuum line to the free end of the holder arranged so as tobe pneumatically separated from the hollow, cylindrical interior of theholder.
 9. A placement unit for electric components and for populatingsubstrates with the components, the placement unit comprising: ahousing, a built-in, rotatable holder for the components and, mountedwithin the housing, a rotary drive for the holder, the holder beingconnected via an axially elastic coupling to the rotary drive in atorsionally rigid manner, wherein the pivot bearing is a cylindrical airbearing in which the holder can be moved axially, and the holder at theend facing the rotary drive can be impinged upon by apressure-adjustable gas in the manner of a pressure piston.
 10. Theplacement unit according to claim 9, further comprising a surroundingventilation slot arranged between an air gap of the air bearing and thesection of the holder impinged upon by the gas.
 11. The placement unitaccording to claim 9, wherein the hollow, cylindrical interior of theholder is pneumatically connected to the pressurized end of the holder.